While both infiltrating macrophages and glomerular cells are crucial in the pathogenesis of lupus nephritis (LN), one of the most common and dangerous clinical manifestations of SLE, their interactions remain incompletely understood. Appreciating the relative contribution of macrophages and resident kidney cells, and their interactions, would help us to understand the pathogenesis of LN and could provide an opportunity for more targeted therapies. Abnormal interaction of TNF-receptor superfamily members and their cognate ligands are believed to play a central role in lupus pathogenesis. Inhibition of several of these ligand/receptor pairs has already shown promise in clinical trials in human lupus. TWEAK is a secreted cytokine member of the TNF-ligand superfamily. TWEAK binds to its receptor, Fn14, and stimulates macrophages, fibroblasts, synoviocytes, and endothelial cells to secrete chemokines, cytokines, and other proinflammatory mediators. Our preliminary results suggest that TWEAK/Fn14 signaling is essential in the pathogenesis of LN: 1. Urinary TWEAK levels are increased in human LN, and correlate with disease activity; 2. The TWEAK receptor, Fn14, is expressed by murine and human mesangial cells (MC) and podocytes in vitro, and in lupus kidneys in vivo. Treatment of podocytes with TWEAK leads to increased expression of key pathogenic cytokines, including MCP-1, RANTES, VCAM-1, and IP-10. Increased MCP-1 in the kidney leads to recruitment of inflammatory cells and amplification of the local inflammatory process; 3. Injection of TWEAK to B6 mice increases MCP-1 expression, promotes chemotaxis of T cells and macrophages into the kidney, and stimulates glomerular cell proliferation; 4. Preliminary studies show a beneficial effect of anti-TWEAK antibodies in reducing proteinuria in the chronic graft versus host murine model of SLE. Our hypotheses are that blocking TWEAK-Fn14 signaling will be beneficial in treatment of murine LN, and that TWEAK activation of both macrophages and resident kidney cells is central in the contribution of this cytokine to inflammatory renal disease in lupus. Our Specific Aims for this proposal are as follows: I. Investigate the direct involvement of TWEAK-Fn14 signaling in the pathogenesis of LN and determine its therapeutic potential by a) characterizing lupus prone MRL-lpr/lpr (MRL-lpr) mice with Fn14 deficiency; and b) administering an anti- TWEAK monoclonal antibody (mAb) to lupus prone mice; II. Characterize the relative contribution of TWEAK activation in macrophages to the pathogenesis of LN; III. Determine the effect of TWEAK signaling in resident kidney cells, and particularly podocytes, in the development of LN, including albuminuria and glomerulosclerosis.